Stable isotope micromethods using combined gas chromatography -mass spectrometry will be applied to the study of amino acid - glucose fuel interrelationships in children, particularly the neonate. Direct, in vivo estimates of alanine and lactate incorporation into glucose, of the metabolic "recycling" of three carbon fragments of glycolysis, and of neonatal hepatic substrate response to exogenous glucose will allow the quantitative dynamic aspects of hepatic glucose output to be determined in the human neonate for the first time. Additional in vivo investigations of individual deuterium, carbon-13, and nitrogen-15 amino acid flux rates in children, and of quantitative flow of amino acid nitrogen to gluconeogenic precursors in related rat skeletal muscle preparations will allow expanded definition of possible amino acid-glucose interactions in the pediatric age group.